Liquid filled lens element, lithographic apparatus comprising such an element and device manufacturing method

ABSTRACT

A lens element, for use in a projection system, includes a concave side. The lens element further includes a membrane and a nozzle, the membrane at least covering the concave side of the lens element. The nozzle is arranged for supplying and/or removing a liquid and/or a gas in between the concave side and the membrane.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid filled lens element, alithographic apparatus comprising such an element and a devicemanufacturing method.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude steppers, in which each target portion is irradiated by exposingan entire pattern onto the target portion at one time, and scanners, inwhich each target portion is irradiated by scanning the pattern througha radiation beam in a given direction (the “scanning” direction) whilesynchronously scanning the substrate parallel or anti-parallel to thisdirection. It is also possible to transfer the pattern from thepatterning device to the substrate by imprinting the pattern onto thesubstrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA (when supported by the lens) ofthe system and also increasing the depth of focus.) Several immersionliquids have been proposed.

In the case of the immersion lithography in which the area between thesubstrate and a last lens of the projection system is filled withliquid, the lens numerical aperture (NA) is limited to one which isdependent on its refractive index (even if the refractive index of theliquid is greater than that of the lens) in order to prevent totalinternal reflection of the projected beam as it passes from the lens tothe liquid on its path to the substrate. This limits the angles in whichthe projected beam can pass safely through the lens and the liquid tothe substrate. One way to overcome this limitations is to make the lensout of a material with a larger refractive index, but none greater thann=1.56 is confirmed as being suitable for use at the time of the filingof this application.

Another way to overcome the limitation of the angles in which theprojected beam can pass safely through the lens and the liquid to thesubstrate is described in U.S. patent application Ser. No. 10/959,403,filed Oct. 7, 2004, in the name of the assigneet of this application.That application proposes to provide a curved lens element, in which thelens-liquid boundary is curved, i.e. a concave hollow lens-liquidboundary, by using a curved lens element. This enables a numericalaperture not to be limited by the refractive index of the material, butby the curvature of the surface of the lens.

In order to use such a curved lens element and an immersion liquid in alithographic apparatus, the space between the curved lens element and asubstrate or substrate table needs to be filled with immersion liquid.

SUMMARY OF THE INVENTION

It is desirable to improve the techniques known from the prior art forfilling the curved lens element with a liquid.

According to an embodiment of the invention, there is provided a lenselement, for use in a projection system, the lens element comprising atleast one concave side, wherein the lens element further comprises amembrane and a nozzle, the membrane at least covering the concave sideof the lens element, and the nozzle being arranged for at least one ofsupplying and removing at least one of a liquid or gas in between theconcave side and the membrane.

According to an embodiment of the invention, there is provided a methodof supplying a liquid to a space in between a lens element of aprojection system and a closing member, the lens element comprising atleast one concave side, the closing member facing the concave side ofthe lens element, wherein the lens element comprises a membrane and anozzle, the membrane at least covering the concave side of the lenselement, and the nozzle being arranged to supply at least one of aliquid or a gas in between the concave side and the membrane, the methodcomprising a) supplying at least one of a liquid or a gas in between theconcave side and the membrane via the nozzle, and b) supplying liquid inbetween the membrane and the closing member.

According to an embodiment of the invention, there is provided a methodof supplying a liquid to a space in between a lens element of aprojection system and a closing member, the lens element comprising atleast one concave side, the closing member facing the concave side ofthe lens element, wherein the lens element comprises a membrane and anozzle, the membrane at least covering the concave side of the lenselement, and the nozzle being arranged to remove air from between theconcave side and the membrane, the method comprising a) supplying liquidin between the concave side and the closing member, and b) extractingair from between the membrane and the closing member.

According to an embodiment of the invention, there is provided amembrane, comprising a first membrane layer and a second membrane layer,attached to each other to form an internal space, the membrane furthercomprising a nozzle, the nozzle being arranged for at least one ofsupplying and removing at least one of liquid or air in between thefirst membrane layer and the second membrane layer.

According to an embodiment of the invention, there is provided a methodfor providing a liquid to a space in between a concave lens element of aprojection system and a closing member, the closing member facing theconcave lens element, the method comprising a) providing a membraneaccording to claim 14 in the space under the concave lens element, b)supplying liquid in the space under the concave side and the closingmember, c) supplying at least one of liquid or gas in between the firstmembrane layer and the second membrane layer via the nozzle, and d)removing the membrane from the space under the concave lens element.

According to an embodiment of the invention, there is provided a methodfor providing a liquid to a space in between a concave lens element of aprojection system and a closing member, the closing member facing theconcave lens element, the method comprising a) supplying liquid in thespace between the concave side and the closing member, b) introducing atube into the space and positioning an end of the tube near the concavelens element, c) apply a suction force to the tube in a direction awayfrom the end of the tube near the concave lens element, and d) removingthe tube from the space.

According to an embodiment of the invention, there is provided a tubefor use in the method according to the above.

According to an embodiment of the invention, there is provided a probehead for use in the method according to the above.

According to an embodiment of the invention, there is provided a methodof filling a space under a lens element with a liquid, where the spaceis limited at a first side by the lens element and at a second side by aclosing member surface and where the liquid is being supplied to thespace via an inlet, such that a liquid front travels through the spaceunder the lens element, wherein a flow rate of the liquid through theinlet is controlled such that the velocity of the liquid front is belowa first velocity v_(mic), and a filling pressure does not exceed athreshold filling pressure such that the space is completely filled withthe liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system used in a prior artlithographic projection apparatus;

FIGS. 4 a and 4 b depict a view from above and a side-view respectivelyof liquid supply system according to another prior art lithographicprojection apparatus;

FIG. 5 depicts a further view of the liquid supply system according to aprior art lithographic projection apparatus;

FIG. 6 depicts a final element of a projection system in a prior artlithographic projection system apparatus;

FIG. 7 depicts an alternative final element of a projection systemaccording to a prior art lithographic projection apparatus;

FIG. 8 depicts a final element of a projection system together with agas entrapment according to an embodiment of the invention;

FIGS. 9 a-d depict a lens element according to an embodiment;

FIGS. 10 a and 10 b depict a membrane according to an embodiment;

FIGS. 11 a-f depict a lens element according to a further embodiment;

FIGS. 12 a-g depict a lens element according to a further embodiment;

FIG. 13 depicts a membrane according to a further embodiment;

FIGS. 14 a-h depict a lens element according to a further embodiment;

FIGS. 15 a-g depict a lens element according to a further embodiment;

FIG. 15 h depicts a probe head according to an embodiment;

FIGS. 16 a and c depict further embodiments;

FIG. 16 b depicts a probe head according to an embodiment;

FIGS. 17 a-d depict probe heads according to further embodiments;

FIG. 18 depict a lens element according to a further embodiment;

FIGS. 19 a-d depict a lens element according to a further embodiment;and

FIGS. 20 a-b depict a top view of a lens element according to a furtherembodiment.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation or EUV radiation). A support (e.g. a mask table) MT isconfigured to support a patterning device (e.g. a mask) MA and isconnected to a first positioner PM configured to accurately position thepatterning device in accordance with certain parameters. A substratetable (e.g. a wafer table) WT is configured to hold a substrate (e.g. aresist-coated wafer) W and is connected to a second positioner PWconfigured to accurately position the substrate in accordance withcertain parameters. A projection system (e.g. a refractive projectionlens system) PL configured to project a pattern imparted to theradiation beam B by patterning device MA onto a target portion C (e.g.comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, todirect, shape, and/or control radiation.

The support supports, e.g. bears the weight of, the patterning device.It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support can usemechanical, vacuum, electrostatic or other clamping techniques to holdthe patterning device. The support may be a frame or a table, forexample, which may be fixed or movable as required. The support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”The term “patterning device” used herein should bebroadly interpreted as referring to any device that can be used toimpart a radiation beam with a pattern in its cross-section such as tocreate a pattern in a target portion of the substrate. It should benoted that the pattern imparted to the radiation beam may not exactlycorrespond to the desired pattern in the target portion of thesubstrate, for example if the pattern includes phase-shifting featuresor so called assist features. Generally, the pattern imparted to theradiation beam will correspond to a particular functional layer in adevice being created in the target portion, such as an integratedcircuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives radiation from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation is passed from the source SO tothe illuminator IL with the aid of a beam delivery system BD comprising,for example, suitable directing mirrors and/or a beam expander. In othercases the source may be an integral part of the lithographic apparatus,for example when the source is a mercury lamp. The source SO and theilluminator IL, together with the beam delivery system BD if required,may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD to adjust the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support (e.g., mask table MT), and ispatterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam B, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

-   1. In step mode, the mask table MT and the substrate table WT are    kept essentially stationary, while an entire pattern imparted to the    radiation beam is projected onto a target portion C at one time    (i.e. a single static exposure). The substrate table WT is then    shifted in the X and/or Y direction so that a different target    portion C can be exposed. In step mode, the maximum size of the    exposure field limits the size of the target portion C imaged in a    single static exposure.-   2. In scan mode, the mask table MT and the substrate table WT are    scanned synchronously while a pattern imparted to the radiation beam    is projected onto a target portion C (i.e. a single dynamic    exposure). The velocity and direction of the substrate table WT    relative to the mask table MT may be determined by the    (de-)magnification and image reversal characteristics of the    projection system PL. In scan mode, the maximum size of the exposure    field limits the width (in the non-scanning direction) of the target    portion in a single dynamic exposure, whereas the length of the    scanning motion determines the height (in the scanning direction) of    the target portion.-   3. In another mode, the mask table MT is kept essentially stationary    holding a programmable patterning device, and the substrate table WT    is moved or scanned while a pattern imparted to the radiation beam    is projected onto a target portion C. In this mode, generally a    pulsed radiation source is employed and the programmable patterning    device is updated as required after each movement of the substrate    table WT or in between successive radiation pulses during a scan.    This mode of operation can be readily applied to maskless    lithography that utilizes programmable patterning device, such as a    programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Different solutions are known to provide a lithographic apparatus usingimmersion techniques. A known system for providing a liquid is to use asystem to provide liquid on only a localized area of the substrate W andin between a final element FE of the projection system PL and thesubstrate W using a liquid confinement system (the substrate W generallyhas a larger surface area than the final element of the projectionsystem PL). One known way to arrange for this is illustrated in FIGS. 2and 3, in which liquid is supplied by at least one inlet IN onto thesubstrate W, preferably along the direction of movement of the substrateW relative to the final element FE, and is removed by at least oneoutlet OUT after having passed under the projection system PL. That is,as the substrate W is scanned beneath the element in a −X direction,liquid is supplied at the +X side of the element and taken up at the −Xside. FIG. 2 shows the arrangement schematically in which liquid issupplied via inlet IN and is taken up on the other side of the elementby outlet OUT which is connected to a low pressure source. In theillustration of FIG. 2 the liquid is supplied along the direction ofmovement of the substrate W relative to the final element, though thisdoes not need to be the case. Various orientations and numbers of in-and out-lets positioned around the final element are possible. Oneexample is illustrated in FIG. 3 in which four sets of an inlet with anoutlet on either side are provided in a regular pattern around the finalelement.

Another solution which has been proposed is to provide the liquid supplysystem with a seal member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. Such a solution is illustrated in FIG. 4. Theseal member is substantially stationary relative to the projectionsystem in the XY plane though there may be some relative movement in theZ direction (in the direction of the optical axis). A seal is formedbetween the seal member and the surface of the substrate. Preferably theseal is a contactless seal such as a gas seal.

According to U.S. patent application Ser. No. 10/959,403, liquid 11 ispumped into one side of the apparatus and out of the other side of theapparatus. As is depicted in FIG. 5, a reservoir 10 forms a contactlessseal to the substrate W around the image field of the projection systemPL so that liquid 11 is confined to fill a space between the substratesurface and the final element FE of the projection system PL. Thereservoir 10 is formed by a seal member 12 positioned below andsurrounding the final element FE of the projection system PL. Liquid 11is brought into the space below the projection system PL and within theseal member 12. The seal member 12 extends a little above the finalelement FE of the projection system PL and the liquid level rises abovethe final element FE so that a buffer of liquid is provided. The sealmember 12 has an inner periphery that at the upper end preferablyclosely conforms to the shape of the projection system PL or the finalelement FE thereof and may, e.g., be round. At the bottom, the innerperiphery closely conforms to the shape of the image field, e.g., whichis rectangular though this need not be the case.

The liquid 11 is confined in the reservoir 10 by a gas seal 16 betweenthe bottom of the seal member 12 and the surface of the substrate W. Thegas seal 16 is formed by gas, e.g. air or synthetic air but preferablyN₂ or another inert gas, provided under pressure via an inlet 15 to agap between seal member 12 and substrate W and extracted via a firstoutlet 14. An overpressure on the gas inlet 15, a substantially vacuumlevel on the first outlet 14 and geometry of the gap are arranged sothat there is a high-velocity gas flow inwards that confines the liquid.

Turning to FIG. 6, the projection system PL comprises a final element20. The final element 20 is most commonly a spherical lens, though itcould be another element such as a diffractive or refractive element.The liquid 11 is supplied between this final element 20 and thesubstrate W. FIG. 6 shows the element 20 as used in prior artlithographic apparatus. It is a planar convex lens.

For all optical rays, there is what is known as a sine-condition:n_(resist) sin θ_(resist)=n_(liquid) sin θ_(liquid)=n_(lens) sinθ_(lens)  (1)

where:

n_(resist)=refractive index of a resist layer R provided on thesubstrate,

n_(liquid)=refractive index of the liquid 11,

n_(lens)=refractive index of the lens,

θ_(lens)=angle between normal at lens-liquid boundary and radiation beamin the lens,

θ_(liquid)=angle between normal at lens-liquid boundary and radiationbeam in the liquid,

θ_(resist)=angle between normal at liquid-resist boundary and radiationbeam in the resist.

Note that formula (1) applies because the lens-liquid boundary and theliquid-resist boundary are substantially parallel. In other words, inorder for an optical ray to pass unaffected through the final element FE(or lens 20 in this case), the liquid 11 and the resist layer R, thisformula needs to balanced. This means that the lowest refractive indexof the lens material, resist layer R or liquid limits the numericalaperture (NA) because:NA=n_(lens) sin θ_(lens)  (2)

If resists and liquids with a refractive index of greater than 1.56 areused, then the sine-condition can not be met at the lens-liquid boundaryand total internal reflection occurs as shown in FIG. 6 with an arrow P.In order to solve this problem, the normal on the surface of thelens-liquid boundary is tilted by using a curved (concave) lens element21. This enables a numerical aperture not to be limited by therefractive index of the lens material, but by the curvature of the lenssurface.

In other words, because the refractive index (n_(lens)) is verydifficult to increase, it is the sin θ_(lens) which must be adjusted inorder to balance formula (1).

Tilting the lens-liquid boundary is achieved by using a lens 21 whichhas a convex face facing the incoming projected patterned beam and aconcave face facing the outgoing projected patterned beam. This may, forinstance, be a meniscus convex lens which has a positive radius ofcurvature at both of its opposing faces. By “positive” radius ofcurvature it is meant that if light were entering from the left, thelens face would be bulging towards the left. If both faces have apositive radius of curvature, the lens would be convex on the left andconcave on the right. Looking at FIG. 7, the light is coming from thetop of the page and so the lens bulges towards the top of the page.

Again, the sine-condition applies:n_(liquid) sin θ_(liquid, 1)=n_(lens) sin θ_(lens)  (2)andn_(liquid) sin θ_(liquid, 2)=n_(resist) sin θ_(resist)  (3)

where:

n_(resist)=refractive index of the resist provided on the substrate,

n_(liquid)=refractive index of the liquid 11,

n_(lens)=refractive index of the lens,

θ_(lens)=angle between normal at lens-liquid boundary and radiation beamin the lens,

θ_(liquid, 1)=angle between normal at lens-liquid boundary and radiationbeam in the liquid,

θ_(liquid, 2)=angle between normal at liquid-resist boundary andradiation beam in the liquid,

θ_(resist)=angle between normal at liquid-resist boundary and radiationbeam in the resist.

The use of liquids with a refractive index which is as high as possibleand therefore higher than that of the lens improves the resolution ofthe pattern image on the substrate W.

To fully exploit the imaging properties in the future, immersion systemsare needed with a liquid 11 having a high refractive index(n_(liquid)>1,6), and curved (concave) lens elements 21 will be used asfinal elements FE. However, when filling the space between the substrateW and the curved (concave) lens element 21 with an immersion liquid, gasentrapment may occur.

The space in between the substrate W and the last lens element, formedby a curved (concave) lens element 21, is filled with a liquid asdescribed above with respect to the prior art. During the filling, forinstance before initial use of the system, a liquid surface level willrise from bottom to top. As a result, gas or air may be entrappedbetween the rising liquid surface level and the curved (concave) lenselement 21. This is schematically depicted in FIG. 8.

FIG. 8 depicts a substrate W that may have a layer of resist R providedon it. Further depicted is the last lens element 21 of the projectionsystem PL, the curved (concave) lens element 21. The hardware forproviding the immersion liquid 11, removing the liquid 11 and keepingthe liquid 11 confined, as described above, are not depicted in FIG. 8.FIG. 8 shows a gas entrapment G (or air pocket) in between the surfacelevel of the immersion liquid 11 and the lens element 21.

Such a gas entrapment G will negatively influence the imaging quality ofthe lithographic apparatus, as it changes the optical characteristics ofthe lithographic apparatus in a negative and unpredictable way.

Therefore, a number of embodiments are described for minimizingundesired gas entrapment G under the curved (concave) lens element 21.

According to an embodiment, the curved (concave) lens element 21comprises a membrane ME that is connected to the concave side of lenselement 21. FIG. 9 a shows a side view of such a lens element 21comprising membrane ME. FIG. 10 a depicts a bottom view of such a lenselement 21. In an embodiment, membrane ME is connected to the lenselement 21 along the circumference of the membrane ME, as is shown inFIGS. 9 a and 10 a. The connection between the membrane ME and the lenselement 21 may be formed in any suitable way, for instance by aconnection seam, e.g. a glued seam GS.

It will be understood that the membrane ME is attached to the lenselement 21 in such a way that the connection seam (glued seam GS) isoutside the imaging area of the lens element 21.

Gluing is a known technique used to fix a pellicle to a reticle mount,mask MA or mask table MT. The lower surface of such a pellicle, i.e. thesurface opposite to the surface to which the pellicle is adhesivelybonded, is usually coated with a pressure-sensitive adhesive based on apolybutene resin, polyvinyl acetate resin, acrylic resin or the like inorder to facilitate mounting of the pellicle. U.S. Pat. No. 5,378,514discloses the use of a hot-melt adhesive. Similar gluing techniques maybe used for attaching the membrane ME to the lens element 21, or gluingdifferent membrane layers together, as will be described in more detailbelow, for instance with reference to FIG. 13.

FIG. 10 b depicts a side view of membrane ME, before it is connected tolens element 21. FIG. 10 b depicts a nozzle NO. At the edges of themembrane ME glue for glued seam GS may be provided to allow easyconnection of the membrane ME to the lens element 21.

The membrane ME may be made of any suitable material. The membrane MEmay for instance be made of a pellicle material. For instance, themembrane ME may be made of fluoropolymers, such as TEFLON AF (trademarkof DuPont), or CYTOP (trademark of Asahi Glass Company). The use ofpellicle material is described in ‘Principles of lithography’ by HarryJ. Levinson, published by SPIE—The international Society for OpticalEngineering (ISBN 0-8194-4045-0).

Optionally an AR (anti-relection) coating is used on the membrane ME,such as metal oxides or fluorides.

The index of the membrane ME may be optimized to be close to the indexof the liquid (n_(membrane)=n_(liquid)±0,3).

The membrane ME may have a thickness of about 250 nm-2 μm, depending onthe circumstances. It should be appreciated that the thickness asselected is a trade off between mechanical strength andabsorption/aberration.

The refractive index is approximately 1.34 for CYTOP and approximately1.31 for Teflon AF. The exact value of the refractive index depends onthe exact composition of the material and the manufacturing process andit is mentioned in the literature that the refractive index can bevaried. The refractive index of the material of membrane ME can bechosen to match with the refractive index of the immersion liquid 11,which for instance is 1.33, to minimize disturbance of the beam PB bythe membrane ME as a result of refraction.

A high refractive index may be chosen of approximately 1.6. In case themembrane is thin enough, e.g. 10 μm, the resulting aberration islimited.

Along the circumference of the membrane ME nozzle NO is provided. Thisnozzle NO may be an inlet nozzle NO for providing liquid, but may alsobe used as an outlet nozzle for removing at least one of liquid, gas andair. The nozzle NO may be connected to appropriate liquid supplies,pumps etc.

FIG. 9 a depicts the initial configuration of the lens element 21 andthe membrane ME before liquid is provided. The membrane ME sticks to thelens element 21. This may be the result of adhesion forces in betweenthe membrane ME and the lens element 21. Also, the membrane ME may besucked against the lens element 21 as a result of underpressure orvacuum in between the membrane ME and the lens element 21.

In a first action, liquid 11.1 is supplied through nozzle NO to fill thespace in between the lens element 21 and the membrane ME. FIGS. 9 b and9 c show respective phases of the filling process. FIG. 9 b shows thephase in which part of the membrane ME is filled with liquid 11.1. FIG.9 c shows the phase in which the membrane ME is totally filled withliquid 11.1.

In a second action, the remaining space in between the membrane ME andthe substrate W or substrate table WT is filled with liquid 11.2 usingfilling techniques and devices known to a skilled person, and of whichsome examples have been discussed above with respect to the prior art.

As a result of the filling process as shown in FIGS. 9 a-d, the totalspace in between the substrate W or substrate table WT and the lenselement 21 is filled with liquid 11.1, 11.2, without a gas entrapment G.

The membrane ME may be very thin in order to minimize absorption and/oraberrations of the radiation beam B. In order to reduce the stress onthe membrane ME and/or to reduce the mechanical force on the connectionbetween the membrane ME and the lens element 21 an alternative fillingprocess may be employed.

According to the alternative, again, the filling process starts with themembrane ME stuck against the lens element 21, as depicted in FIG. 11 a.Then, the space in between the substrate W (or substrate table WT) andthe membrane ME is filled with liquid 11.2 using filling techniques anddevices known to a skilled person, and of which some examples have beendiscussed above with respect to the prior art. This is depicted in FIG.11 b. When this is finished, a gas entrapment G will be present inbetween the membrane ME and the surface of the liquid 11.2, as isdepicted in FIG. 11 b.

Next, liquid 11.1 is supplied via nozzle NO to fill the space in betweenthe lens element 21 and the membrane ME. As a result, some of the liquid11.2 in between the membrane ME and the substrate W or substrate tableWT will be pushed out. This phase is depicted in FIG. 11 c.

Also, the gas entrapment G will be pushed out by the liquid 11. 1,leaving the total space in between the lens element 21 and the substrateW or substrate table WT filled with liquid, without a gas entrapment G.This is depicted in FIG. 11 d.

According to this alternative, the stress on the membrane ME and/or themechanical force on the connection between the membrane ME and the lenselement 21 is reduced, as during the filling process the pressure onboth sides of the membrane ME is more balanced than in the fillingprocess discussed with reference to FIGS. 9 a-d. For instance, in thephase depicted in FIG. 9 c, in which the total space in between the lenselement 21 and the membrane ME is filled with liquid 11.1, while thespace in between the membrane ME and the substrate W or substrate tableWT is empty, the stress on the membrane ME and/or to the mechanicalforce on the connection between the membrane ME and the lens element 21are relatively high. This is prevented in the alternative embodimentdescribed with respect to FIGS. 11 a-d.

According to a further alternative, the stress on the membrane ME and/orthe mechanical force on the connection between the membrane ME and thelens element 21 may be reduced by deflating the membrane ME after thefilling process described with reference to FIGS. 11 a-d. This may bedone by pumping liquid 11.1 present in between the membrane ME and thelens element 21 after the filling process described with reference toFIGS. 11 a-d out via nozzle NO and, simultaneously, pumping liquid 11.2into the space between the membrane ME and the substrate W or substratetable WT, to replace the liquid that is pumped out. This is depicted inFIG. 11 e. The membrane ME will stick to the lens element 21 again, asdepicted in FIG. 11 f.

As a result, the membrane ME is stuck to the lens element 21. Thisminimizes the mechanical disturbance of the lens element 21. Also, themembrane ME is moved further out of focus, reducing its influence on theimaging.

It should be appreciated that the actions depicted in FIGS. 11 e and 11f may also be employed after the embodiment described with reference toFIGS. 9 a-d.

It should be appreciated that according to this variant, since themembrane is emptied, the membrane ME may be filled with a differentliquid 11.1 than the space outside the membrane ME. In fact, themembrane ME may be filled with air or an appropriate gas.

The membrane ME may be relatively thin and may easily be damaged. Also,the membrane ME may be made of a material known from pellicles. Becauseof this, the membrane ME may degrade, for instance as a result ofexposure to the radiation beam (e.g. ultraviolet, deep ultraviolet(DUV). If the membrane ME is attached to the lens element 21,replacement may be difficult and time consuming.

According to an alternative, a temporary membrane ME is used, which maybe removed and replaced easily. In fact, the membrane ME may be removedafter each filling process, before the first exposure, and is thereforenot exposed to the beam PB. This alternative will be further explainedwith reference to FIGS. 12 a-12 g.

In a first action, the space in between the substrate W or substratetable WT and the lens element 21 is filled with liquid 11.2. This isdepicted in FIG. 12 a. During this action, no membrane ME is present.When this is finished, a gas entrapment G will be present in between thelens element 21 and the surface of the liquid 11.2, as is depicted inFIG. 12 a.

In a next action, a membrane ME′ is brought into the space in betweenthe lens element 21 and the substrate W or substrate table WT. Themembrane ME′ is different from the membrane ME described above, as it isa double layer membrane ME′, instead of a single layer membrane ME. Thisis further explained below with reference to FIG. 13.

Next, the space in between the two layers of the membrane ME′ is filledwith liquid 11.1 using nozzle NO. As a result, part of the liquid 11.2already present in between the lens element 21 and the substrate W orsubstrate table WT, as well as the gas entrapment G is pushed out. Thisis depicted in FIG. 12 c.

The membrane ME′ is formed in such a way, that it will fill the completespace in between the lens element 21 and the substrate W or substratetable WT, or at least the concavity under the lens element 21. Thisensures that the entire gas entrapment is pushed out. This is depictedin FIG. 12 d.

Optionally, in a next action, the liquid 11.1 inside the membrane ME′ ispumped out of the membrane ME′, until there is no liquid 11.1 present init anymore. Simultaneously, the space in between the substrate W orsubstrate table WT and lens element 21 may be filled with liquid 11.2 toreplace the liquid 11.1 that is pumped out of membrane ME′. Theseactions are depicted in FIGS. 12 e and 12 f.

Finally, the deflated membrane ME′ may be removed (see FIG. 12 g), forinstance before exposure.

The alternative embodiment described with reference to FIGS. 12 a-gprovides a filling technique and membrane ME′ that allows easyreplacement. In fact, the membrane ME′ may be removed before exposure,minimizing degrading of the membrane ME′ as a result of the exposure.Removal of the membrane ME′ before exposure, ensures that the membraneME′ has no negative influence on the imaging process, for instance as aresult of refraction.

It will be understood that according to this variant, since the membraneME′ is emptied, the membrane ME′ may be filled with a different liquid11.1 than the space outside the membrane ME′. In fact, the membrane ME′may be filled with air or an appropriate gas.

FIG. 13 depicts a side view of a double layer membrane ME′. The membraneME′ comprises a first membrane layer ML1 and a second membrane layer ML2that are glued together at their circumference, for instance by a glueseam GS. FIG. 13 further depicts nozzle NO. For the membrane ME′, thesame materials may be used as for the single layer membrane ME describedwith reference to FIG. 10 b.

According to a further alternative embodiment, the membrane may be asubstantially rigid membrane ME″, having a rigid shape corresponding tothe shape of the concavity of the lens element 21. The membrane ME″ mayfurther comprise perforations PE, as depicted in FIGS. 14 a and 14 b,depicting a side view and a top view respectively. The membrane ME″ mayfurther comprise a nozzle NO, arranged to be connected to an appropriatepump, or the like.

The membrane ME″ may further comprise a glue seam GS, for connecting themembrane ME″ to the lens element 21, as shown in FIG. 14 e.

FIGS. 14 e, f and g show respective actions for filling the space inbetween the lens element 21 and the substrate W or substrate table WT.In a first action, the membrane ME″ is positioned such that it isrelatively close to the surface of the lens element 21 facing thesubstrate W or substrate table WT. In between lens element 21 and themembrane ME″ is a small opening. This is depicted in FIG. 14 e.

Next, the space under the lens element 21 is filled with liquid 11. Thisis depicted in FIG. 14 f. The filling results in a gas entrapment Gpresent, as also shown in FIG. 14 f.

Then, the gas entrapment G is extracted via nozzle NO, as shown in FIG.14 g. Liquid 11 may flow through the perforations PE to replace the gasentrapment G. During this, it may be necessary to add extra liquid 11 toreplace the gas entrapment G. This finally results in the space inbetween the lens element 21 and the substrate W or substrate table WTbeing completely filled with liquid, without a gas entrapment G.

According to a further variant, the membrane ME″ may comprise at leastone spacer SP, as shown in FIGS. 14 c and 14 d depicting a side view anda top view respectively. The spacer SP prevents the membrane ME″ frombeing sucked against the lens element 21 during gas extraction. Thespacers SP may be made of the same material as the membrane ME″ and mayfor instance be made of pellicle material as described above.

The membrane ME″ may be rigid for instance by making it thicker and/orspacers SP may be added, so that it does not collapse during gasextraction. The membrane ME″ then keeps the same curvature as the curvedlens element. By using perforations, the gas/air may be extracted untilthe area fills with liquid.

The size, shape and distribution of the perforations PE may be varied tovary the effectiveness of the extraction of the gas. If pieces of thespacer SP are used they may also be varied in size shape anddistribution.

The material of the spacers SP and membrane ME″ may be chosen such thattheir refractive index is equal to that of the liquid, to minimizedisturbance of the beam PB.

According to a further embodiment, the gas entrapment G as it maydevelop when filling the space between the last lens element 21 and thesubstrate W or substrate table WT may be removed by introducing a tubeTU. One end of the tube TU may be positioned in the gas entrapment G,while the other end is connected to an appropriate suction device, suchas a pump PU. The pump PU may then be turned on, sucking the gas out ofthe gas entrapment G. When all gas is sucked out of the gas entrapmentG, the tube TU and the pump PU will start sucking liquid. In a nextaction, the tube TU is removed.

In a first action, the space in between the substrate W or substratetable WT and the membrane ME is filled with liquid 11. This is depictedin FIG. 15 a. When this is finished, a gas entrapment G will be presentin between the membrane ME and the surface of the liquid 11.

In a further action, the tube TU is introduced in the space in betweenthe substrate W or substrate table WT and the lens element 21. The endof the tube TU is positioned such that it is in the gas entrapment G.The end of the tube TU may be positioned as high as possible in the gasentrapment G. The end of the tube TU may be positioned against the lenselement 21 or at a small distance from it, to prevent damaging the lenselement 21. This is depicted in FIGS. 15 b and 15 c. The tube TU may(partially) be made of Teflon.

In order to position the tube TU, it may be provided with joints,extendable parts, etc. The other end of the tube TU is connected to thepump PU.

In a further action, the pump PU sucks the gas out of the gas entrapmentG, as a result of which the liquid surface level will rise. The pump PUmay possibly also suck liquid 11 once the gas entrapment G hasdisappeared. This is shown in FIG. 15 d.

In a final action, the tube TU is removed again, as depicted in FIGS. 15e and 15 f, leaving the space in between the lens element 21 and thesubstrate W or substrate table WT completely filled with liquid 11.

According to a variant, the tube TU may be provided with a probe headPH, see FIG. 15 g. The probe head PH may be formed to match the shape ofthe concave surface of the lens element 21. This ensures that the gasentrapment G is sucked out efficiently.

The probe head PH may (partially) be made of Teflon.

The part of the probe head PH matching the concave surface of the lenselement 21 may be provided with a number of perforations PE. This isschematically depicted in FIG. 15 h, showing a top view of the probehead PH. It should be appreciated that the probe head PH may have anysuitable shape and number of perforations PE.

According to a variant, pump PU may be used to pump liquid or airtowards the lens element 21, instead of being used as a suction deviceto pump air or liquid from the lens element 21. This may be done withthe tube TU with or without the probe head PH. The tube TU and the pumpPU may then be used as a ‘sprinkler’ to clean lens element 21.

For instance, if there is a drying stain on the lens element 21, forinstance as a result of prior use in combination with immersiontechniques, the pump PU may use a cleaning solution different to theimmersion liquid 11 to remove the drying stain.

Cleaning of the lens element 21 may be applied on an immersed lenselement 21 (liquid 11 is present), or may be applied on a dry lenselement 21. The tube TU and the pump PU may also be used to introduceair or gas, for instance dry and/or warm air/gas, for drying the lenselement 21 after cleaning or immersion.

In case the tube TU is provided with a probe head PH, the probe head PHmay be formed as the probe head PH depicted in FIGS. 15 g and 15 h.According to an alternative, the probe head PH is formed as depicted inFIGS. 16 a and 16 b. According to this variant, the probe head PH isformed to follow the concave surface of the lens element 21, to ensureproper cleaning of the imaging surface of the lens element 21.

The cleaning procedure may comprise a first action of inserting the tubeTU and probe head PH and positioning it with respect to lens element 21.In a next action, the cleaning may be carried out by spraying the lenselement 21 with a cleaning liquid. The cleaning action may furthercomprise further spraying the lens element 21 with a rinsing agent, suchas water. The cleaning action may further comprise spraying the lenselement 21 with a rinsing agent, such as acetone. After this, thecleaning action may comprise drying the lens surface by introducing gasor air, for instance dry and/or warm air/gas. In a final action, thetube TU and probe head PH may be removed.

According to a variant, the cleaning procedure may be performed on a‘wet’ lens element, i.e. in a situation in which the space under thelens is filled with immersion liquid 11. The liquid 11 may be applied inany suitable way, for instance according to any one of the fillingtechniques described in this document.

In a next action, the tube TU and the probe head PH are positioned asexplained above. In case tube TU and possibly probe head PH is stillpositioned as a result of a previous filling procedure, the tube TU andprobe PH already in position may be used for the cleaning procedure.However, the tube TU and probe head PH may also be replaced with another tube TU and/or probe head PH.

In a further action, the lens element 21 is sprayed with a cleaningliquid, or a subsequent combination of cleaning liquids. When this isdone, the liquid 11 present under the lens element 21 may be refreshed.Once the cleaning is finished, the lens element 21 may be sprayed withimmersion liquid 11 via tube TU and possibly probe head PH, while thealready present immersion liquid 11 is refreshed. This is done until theconcentration of cleaning liquid in the immersion liquid 11 is below aparticular concentration. In a final action, the tube TU and the probehead PH are removed.

According to a further variant, two tubes TU are provided, where a firsttube TU1 is used to provide cleaning liquid to the lens element 21 and asecond tube TU2 is used to remove liquid from the lens element 21. Bothtubes TU1, TU2 may be connected to an appropriate pump PU. This isdepicted in FIG. 16 c.

By simultaneously spraying and extracting liquid, a flow along thesurface of lens element 21 is provided, improving the cleaningprocedure.

It should be appreciated that the probe head PH may be given anysuitable shape. FIG. 17 a, e.g. shows a probe head PH′ that isdisc-shaped to increase the cleaning of the entire lens element 21. FIG.17 a depicts a top view of such a probe head PH′ comprising a pluralityof perforations PE.

FIG. 17 b shows an alternative probe head PH″. The probe head PH″ maycomprise two or more compartments CO, that each may have there own tubeTU connected to it. FIG. 17 b depicts a top view of probe head PH″ withtwo compartments CO and two tubes TU. Providing a probe head PH with twoor more compartments CO ensures a proper flow of cleaning liquid to theentire surface of lens element 21.

The probe head PH may be given any alternative shape, for instancecorresponding to the surface of the lens element 21. The probe head PHmay also be used to clean lens elements 21 having a more or less concaveshape or even a flat or convex shape.

FIG. 17 c depicts a side view of probe head PH′″, having a substantiallyflat shape. The probe head PH′″ further comprises three compartments CO,each having their own tube TU connected to it.

Alternatively, the probe head PH may be split in more than onecompartments to create a rotating flow of cleaning fluid. For instance,the probe head may be provided with a central portion and a plurality ofouter compartments, as is schematically depicted in FIG. 17 d, showing atop view of such a probe head PH. As is schematically depicted, thecentral portion is extracted and the outer compartments are separatelyprovided with liquid. This way, a rotating flow may be generated.

According to a further embodiment, the development of a gas entrapment Gmay be prevented by providing a modified lens element 21, as depicted inFIG. 18. According to this embodiment, lens element 21 is provided withan additional plate P, closing the concavity under lens element 21. Thespace in between the plate P and the lens element 21 is filled withliquid 11.1. This may be done when manufacturing lens element 21.

The space in between the plate P and the substrate W or substrate tableWT may be filled with liquid 11.2.

The plate P may be flat, such that it has minimal influence on the beamPB. Also, the plate PL may be made of a material having a refractiveindex that is close to or substantially matches the refractive index ofthe liquid. The plate P may further be relatively thin, to absorb aslittle radiation as possible.

Also, problems associated with turbulence and temperature gradients arereduced. The plate PL may help to create a laminar flow, which resultsin a uniform temperature distribution in the liquid, and thus a uniformindex distribution. This helps to increase the optical performance.

According to a further embodiment, a further technique is introduced forfilling the space under curved lens element 21 with a liquid 11.According to this embodiment, the space in between the lens element 21and the substrate W or substrate table WT is filled by letting capillaryforces push the gas/air out of the concavity of the lens element 21. So,according to this embodiment, the space under the lens element 21 isfilled in such a way that no gas entrapment G will occur.

The capillary filling technique is schematically depicted in FIGS. 19a-d. FIG. 19 a shows a substrate W above which a lens element 21 ispositioned and a liquid inlet IN. Liquid 11 is supplied via inlet IN tofill the space under lens element 21. FIGS. 19 a-d show subsequentphases of the capillary filling procedure. The liquid enters the spaceunder lens element 21. A liquid front LF progresses from the inlet INinto the space under lens element 21, until it is completely filled.

As depicted in FIGS. 19 a-c, the liquid front LF is hollow as a resultof the adhesive forces between the liquid 11 and the surface of lenselement 21 and the adhesive forces between the liquid 11 and thesubstrate or substrate table WT. As a result of this adhesive forces orcapillary forces, the liquid front LF bridges the gap between the lenselement 21 and the substrate W or substrate table WT, thereby preventingdevelopment of a gas entrapment G.

In a first action, the filling is started by letting liquid 11 enter thespace under the lens element 21 via inlet IN. This is shown in FIG. 19a.

Next, the capillary forces push the meniscus or liquid front LF betweenthe lens element 21 and the substrate W or substrate table WT. This isshown in FIG. 19 b.

After a certain time, the liquid front LF reaches the other side of thelens element 21, as depicted in FIG. 19 c. Finally, the whole spaceunder lens element 21 is filled with liquid 11, without gas entrapmentG.

The success of such a capillary filling technique depends on a number ofparameters, such as the height h between the lens element 21 and thesubstrate W or substrate table WT, the kind of liquid 11 used, thecontact angle between the liquid 11 and the lens element 21 and betweenthe liquid 11 and the substrate W or substrate table WT.

For instance, the capillary filling technique is possible up to amaximum height h_(max) between the lens element 21 and the substrate Wof substrate table WT, where the surface tension of the liquid 11 is nolonger able to maintain the meniscus or liquid front LF due togravitational forces.

The (advancing) contact angle of the lens element 21 and the substrate Wor substrate table WT is also of importance. The angle θ_(lens) is theangle through the liquid from lens to meniscus. The angle θ_(CLD) is theangle through the liquid from closing member to meniscus, where theclosing member may be a substrate W, substrate table WT, or closingdisk. These angles are depicted in FIG. 19 b.

More hydrophilic (or higher affinity with the immersion liquid, notrestricted to water) means higher capillary forces and thus a largerheight h_(max) that may be bridged. Roughly the relationship betweenliquid and surface properties and the maximum gap h_(max) is given by:

$\left. \left. \begin{matrix}{p_{cap} = \frac{\gamma \cdot \left( {{\cos\left( \theta_{LENS} \right)} - {\cos\left( \theta_{CLD} \right)}} \right)}{h}} \\{p_{g} = {\rho \cdot g \cdot h}}\end{matrix} \right\}\Rightarrow{h \approx \sqrt{\frac{\begin{matrix}{\gamma \cdot \left( {{\cos\left( \theta_{LENS} \right)} -} \right.} \\\left. {\cos\left( \theta_{CLD} \right)} \right)\end{matrix}}{\rho \cdot g}}} \right.,$wherein P_(cap) capillary pressure,

-   -   γ fluid surface tension,    -   θ_(lens) advancing contact angle of the fluid on the lens        element 21,    -   θ_(CLD) the advancing contact angle of the fluid on the closing        member surface, such as substrate W or substrate table WT,    -   h distance between lens element 21 and substrate W,    -   P_(g) gravitational pressure,    -   ρ fluid density,    -   g gravitational constant.

When the liquid 11 is water, and the contact angles (θ_(lens), θ_(CLD))up to 60°, the maximum height h_(max) is approximately 2.7 mm. Fordecalin (surface tension 30 mN/m, density 900 kg/m³) the calculationcomes to a maximum height of h_(max) approximately 1.8 mm.

Liquid 11 is supplied via inlet IN to fill the space under lens element21. In order to successfully fill the space under lens element 21 thefilling velocity v_(LF) of the liquid front LF, and consequently, thefilling flow rate through inlet IN, should not be too high. In case theliquid 11 is supplied too fast, the hollow liquid front LF may bedisturbed and may ‘break’ or ‘collapse’. Once this happens, the liquidfront LF no longer bridges the gap between the lens element 21 and thesubstrate W or substrate table WT and air entrapment G may be formed.

The velocity v_(LF) of the liquid front LF should be chosen according totwo criteria:

1) microscopic collapse, and 2) pressure collapse, which will both beexplained in more detail below.

If the velocity v_(LF) of the liquid front LF is too high, the bulk ofthe liquid 11 may overtake the advancing meniscus or liquid front LF. Asa result the concave liquid front LF may become less concave or evenconvex, causing the liquid front LF to collapse.

Therefore, the velocity v_(LF) of the liquid front LF may be chosen insuch a way that the velocity v_(LF) of the liquid front LF doesn'texceed the advancing meniscus or liquid front LF velocity, given byVionov's equation:

${v_{vionov} = {\frac{1}{A}\frac{\gamma}{\eta}\left( {\theta_{{dy}\; n}^{3} - \theta_{stat}^{3}} \right)}},$wherein V_(vionov) velocity of advancing meniscus or liquid front LF,

-   -   A typical parameter for stable performance, for water A may        typically be 250, for decalin 100,    -   γ fluid surface tension,    -   η viscosity,    -   θ_(dyn) maximum advancing dynamic contact angle,    -   θ_(stat) maximum advancing static contact angle of both surfaces        i.e. lens element 21 and closing member surface.

A is a typical value for stable performance for water, A=250 is expectedfor water, A=100 is expected for decalin.

θ_(dyn) is the maximum advancing dynamic contact angle that can bereached before the liquid front LF or meniscus “trips” and airinclusions or gas entrapments occur. Theoretically θ_(dyn) is 180°, forrobustness usually a lower value is taken, e.g. 120°. This is done so itcan deal with artefacts like scratches on the surface or contaminations(particles, deposits, etc.).

θ_(stat) is the static advancing contact angle, the angle the advancingmeniscus or liquid front makes with a surface just before it starts tomove. These values can be measured with dynamic contact angle measuringdevices, e.g. the Krüss Drop Shape Analysis System (DSA10, DSA100s), aswill be understood by a skilled person. The θ_(static) for the lensmaterial is referred to as θ_(lens), for the closing member (e.g.substrate W or substrate table WT) as θ_(CLD).

The velocity v_(LF) of the liquid front LF and consequently, the fillingflow rate of the liquid 11 through inlet IN, are to be chosen such thatthe velocity of the liquid front LF v_(LF), as indicated in FIG. 19 b ,doesn't exceed the velocity v_(vionov) based on Vionov's equation.

For instance, when the liquid 11 is water, this results in:

Material θ_(max) = θ_(max) = θ_(stat) lens element 21 180° θ_(max) =150° θ_(max) = 120° 90°  50° Quarts 8.9 m/s 5.0 m/s 2.5 m/s 0.1 m/s 130°Teflon 6.8 m/s 3.0 m/s 0.4 m/s —

For instance, when the liquid 11 is decaline (γ=30 mN/m and η≈30 mPa·s), this results in:

Material lens θ_(max) = θ_(stat) element 21 180° θ_(max) = 150° θ_(max)= 120° θ_(max) = 90°  0° Kwarts 1.1 m/s 0.6 m/s 0.3 m/s 0.1 m/s 60°Teflon 1.0 m/s 0.6 m/s 0.3 m/s —

The choice of material of the substrate W or substrate table WTdetermines the highest (static and thus the speed of microscopiccollapse.

Microscopic collapse may occur on both contact surfaces, so on the lastlens element (made of quartz or LuAG) and on the closing member(substrate, substrate table, closing member, made of glass, silicon,silicon carbide, ZERODUR, ULE, etc, possibly coated with teflon).

So, based on the above, the velocity of the liquid front v_(LF) shouldbe kept below a first velocity vmicro, where v_(micro)=v_(vionov).

FIG. 20 a shows a top view of a filling process. The figure depicts anozzle NO supplying liquid 11 to the space under the lens element 21.The figure further shows the subsequent positions of the liquid front LFat subsequent moments in time LF.1, LF.2, LF.3, LF.4.

A suitable filling speed may be computed, based on a meniscus overruncriterion, i.e. the filling pressure p_(fill) should not be too high, asit will destroy the meniscus. The mathematics then becomes:p _(cap) =p _(fill) +p _(grav) +Δp _(visc) +p _(inertia)

This formula generally expresses that the filling pressure p_(fill) isto be balanced with the other pressure terms mentioned, to prevent themeniscus from being overrun. The other pressure terms are a capillarypressure p_(cap), a gravitational pressure p_(grav), a viscous pressurep_(visc) and a inert pressure term p_(inertia). The different pressureterm will be explained in more detail below:

${p_{cap} = {\frac{\gamma}{h}\left( {{\cos\;\theta_{{lens}{(v)}}} + {\cos\;\theta_{{CD}{(v)}}}} \right)}},$

wherein γ represents the surface tension, h the local gap height,θ_(lens(v)) the dynamic contact angle on lens at meniscus speed v, andθ_(CD(v)) the dynamic contact angle on lens at closing member speed v.θ_(xxx(v)) may be computed using Vionov's equation:

${\left( {\theta_{d\;{yn}}^{3} - \theta_{stat}^{3}} \right) = {\frac{A\;\eta}{\gamma}v}},$

with A˜250 for water, ˜100 for high refractive index oils like decalin;p_(grav)=ρgh,

where ρ represents the density of the liquid, g equals 9.81 m/s² and his the gap height, as indicated in FIG. 20 b;

${\Delta\; p_{visc}} = \frac{C_{1}\eta\; x}{h^{3}Q}$where, C₁ represents a constant depending on flow pattern, η theviscosity of the liquid and Q the flow per unit area;p_(inertia)=c₂ρv²,where C₂ is a constant depending on flow pattern.

Based on the above a threshold filling pressure may be computed. Theactual filling pressure pfill should not exceed this threshold fillingpressure to prevent gas entrapments to occur.

Furthermore the flow balance linking the flow into the movement speed ofthe meniscus and the flow velocity distribution over gap height needs tobalanced. The pressure balance has to be fulfilled on all heights. Ifnot, the meniscus locally buckles and air inclusions occur.

It should be appreciated that in order to fulfil both criteria discussedabove, the filling fluid rate through inlet IN needs to be controlled insuch a way that the velocity of the liquid front LF V_(LF), being pushedforwards by the liquid 11 coming in via inlet IN, doesn't exceed maximumvelocity v_(max) as determined according to the above. In order to dothis, the fluid rate through the inlet IN may be varied during thefilling process. It should be appreciated that the maximum allowablefilling rate may be inversely proportional to the size of the liquidfront LF.

When the filling has just commenced, the liquid front LF is relativelysmall, as can be seen in FIG. 20 a (LF. 1). In that case, the fluid ratethrough inlet IN should be kept relatively low. When the filling ishalfway, the liquid front LF is relatively large (LF.3), as can be seenin FIG. 20 a. In that case, the fluid rate may be higher.

It should be appreciated that the filling method as described above isnot restricted to filling hollow lens elements 21, but may also be usedfor fillings space under flat lens elements 21 or convex lens elements21.

This text above refers to the space in between lens element 21 and thesubstrate W or substrate table WT. It should be appreciated that insteadof a substrate W or substrate table WT any closing member may be used.After the space under lens element 21 has been filled with a liquidaccording to any of the embodiments described above, the closing membermay be replaced by a substrate W or substrate table WT. In fact, aspecial closing member may be chosen, made of a specific material,having material features that are suitable for relative quick filling.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. It should be appreciated that, in the context of suchalternative applications, any use of the terms “wafer” or “die” hereinmay be considered as synonymous with the more general terms “substrate”or “target portion”, respectively. The substrate referred to herein maybe processed, before or after exposure, in for example a track (a toolthat typically applies a layer of resist to a substrate and develops theexposed resist), a metrology tool and/or an inspection tool. Whereapplicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm), aswell as particle beams, such as ion beams or electron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lens element, for use in a projection system, the lens elementcomprising: a concave side; a membrane attached to the lens element; anda nozzle, wherein the membrane covers the concave side, and the nozzleis configured to supply a liquid and/or a gas in between the concaveside and a side of the membrane such that the supplied liquid and/or gascontacts the side and causes the membrane to displace from the concaveside without application of a suction force on the other side of themembrane.
 2. A lens element according to claim 1, wherein the membraneis made of a pellicle material.
 3. A lens element according to claim 2,wherein the membrane is made of a fluoropolymer.
 4. A lens elementaccording to claim 1, wherein the lens element is made of a materialhaving a refractive index between 1 and 1.7.
 5. A lens element accordingto claim 1, wherein the membrane is attached to the lens element by aglue seam.
 6. A lens element, for use in a projection system, the lenselement comprising: a concave side; a membrane; and a nozzle, whereinthe membrane covers the concave side, the nozzle is configured to supplyand/or remove a liquid and/or a gas in between the concave side and themembrane, and the membrane is rigid, permeable and substantially followsthe concave side of the lens element.
 7. A lens element according toclaim 6, wherein the membrane comprises perforations.
 8. A lens elementaccording to claim 6, wherein the membrane comprises a spacer at a sidefacing the concave side of the lens element configured to prevent themembrane from sticking against the concave side of the lens element. 9.A method of supplying a liquid to a space in between a lens element of aprojection system and a movable closing member, the lens elementcomprising a concave side, the movable closing member facing the concaveside of the lens element, wherein the lens element further comprises amembrane, the membrane covering the concave side of the lens element,the method comprising: supplying a first liquid and/or a gas in betweenthe concave side and the membrane via a nozzle of the lens element suchthat the first liquid and/or gas contacts both the concave side and themembrane; and supplying a second liquid in between the membrane and themovable closing member such that the second liquid contacts both themembrane and the movable closing member.
 10. A method according to claim9, wherein supplying the second liquid is done after supplying the firstliquid and/or gas.
 11. A method according to claim 9, wherein supplyingthe first liquid and/or gas is done after supplying the second liquid.12. A method according to claim 9, wherein the method further comprisesremoving the first liquid and/or gas from between the concave side andthe membrane after supplying the first liquid and/or gas and aftersupplying the second liquid.
 13. A method according to claim 9, whereinthe first liquid and the second liquid are different.
 14. A method ofsupplying a liquid to a space in between a lens element of a projectionsystem and a movable closing member, the lens element comprising aconcave side, the movable closing member facing the concave side of thelens element, the lens element further comprising a membrane, themembrane covering the concave side of the lens element, the methodcomprising: supplying a liquid in between the concave side and themovable closing member such that the liquid contacts both the concaveside and the movable closing member; and extracting, by a suction force,air from between the membrane and the movable closing member.
 15. Amethod for providing a liquid to a space in between a concave lenselement of a projection system and a closing member, the closing memberfacing the concave lens element, the method comprising: a) providing amembrane in a space under the concave lens element, the membranecomprising a first membrane layer and a second membrane layer, the firstmembrane layer and the second membrane layer being attached to eachother to form an internal space and a nozzle connected to the firstmembrane layer and the second membrane layer, the nozzle configured tosupply and/or remove a liquid and/or a gas in between the first membranelayer and the second membrane layer; b) supplying a first liquid in thespace to contact at least part of the membrane and the closing member;c) supplying a second liquid and/or gas in between the first membranelayer and the second membrane layer via the nozzle; and d) removing themembrane from the space under the concave lens element.
 16. A methodaccording to claim 15, further comprising removing the second liquidand/or gas from between the first membrane layer and the second membranelayer via the nozzle.
 17. A method according to claim 15, wherein a),b), c) and d) are done sequentially.
 18. A method according to claim 15,wherein a), c), b) and d) are done sequentially.
 19. A method accordingto claim 15, wherein the first liquid and the second liquid aredifferent.
 20. A method for providing a liquid to a space in between aconcave, last lens element of a projection system and a movable closingmember, the movable closing member facing the concave lens element, themethod comprising: a) supplying liquid in the space between the concaveside and the movable closing member such that the liquid contacts boththe concave side and the movable closing member; b) introducing a tubeinto the space and positioning an end of the tube near the concave lenselement; c) applying a suction force to the tube in a direction awayfrom the end of the tube near the concave lens element; and d) removingthe tube from the space.
 21. A method according to claim 20, wherein thetube is provided with a joint and/or an extendable part to position theend of the tube near the concave lens element.
 22. A method according toclaim 20, wherein the tube is provided with a probe head shapedaccording to the concave lens element.
 23. A method of filling a spaceunder a lens element with a liquid, wherein the space is limited at afirst side by the lens element and at a second side by a closing membersurface and the liquid is supplied to the space via an inlet such that aliquid front travels through the space under the lens element, themethod comprising: controlling a flow rate of the liquid through theinlet such that a velocity of the liquid front is below a first velocitythat would cause the liquid front to collapse, and a filling pressuredoes not exceed a threshold filling pressure that would cause the liquidfront to collapse, such that the space is completely filled with theliquid.
 24. A method according to claim 23, wherein the first velocityis given by${v_{vionov} = {\frac{1}{250}\frac{\gamma}{\eta}\left( {\theta_{\max}^{3} - \theta_{stat}^{3}} \right)}},$wherein γ is fluid surface tension, η is viscosity, θ_(max) is maximumadvancing dynamic contact angle, and θ_(stat is) maximum advancingstatic contact angle of a surface of the lens element and the closingmember surface.
 25. A method according to claim 23, wherein thethreshold filling pressure p_(fill) is given byp _(cap) =p _(fill) +p _(grav) +Δp _(visc) +p _(inertia), whereinp_(cap) is a capillary pressure, p_(grav) is a gravitational pressure,p_(visc) is a viscous pressure p_(visc) and p_(inertia) is an inertpressure term.